Galerucella species
There is little we can tell you about the species in the picture. The only fact we are sure about is that this is a beetle in the genus Galerucella. In Holland and Britain there are some six species within this genus:
Galerucella calmariensis, the Black-margined Loosestrife Beetle.
Galerucella grisescens, which has no common name in English. It lives on various Knotweeds.
Galerucella pusilla, the Golden Loosestrife Beetle.
Galerucella tenella, the Strawberry Leaf Beetle.
Galerucella lineola, the Brown Willow Beetle.
Galerucella nymphaea, the Water Lily Leaf Beetle.
The one in this picture probably is either the Black-margined Loosestrife Beetle (Galerucella calmariensis) or the Golden Loosestrife Beetle (Galerucella pusilla). These two species are extremely hard to tell apart. Usually they are referred to simply as Loosestrife Beetle. Both species are brownish and even have the same host plant Loosestrife (Lythrum salicaria). Their life cycle is identical as well. The larva starts of by making mines in the leaf of the host plant. Soon it becomes to big to live inside a leaf and it moves to the underside. It does not make a hole in the leaf. The upper layer (the skin, if you want) remains completely intact. But this layer is very thin and it appears you can look through. This type of damage is referred to as windows in a leaf. Many Leaf Fleas also cause window damage to host plants. Loosestrife has been imported in the USA. As it didn't occur there naturally, it had no enemies. It therefor soon became a pest. Both Loosestrife Beetle species were imported from Europe and did a good job in keeping the plant under control.
